1. Field of the Invention
The present invention relates to a method for permanent redundant transmission of data messages in communication systems. The present invention further relates to a communication system for permanent transmission of data messages and to a user of such a communication system.
2. Description of Related Art
A communication system is a system with a plurality of users, which are interconnected by network connections for the mutual exchange of data or the mutual transmission of data. The data to be transmitted are dispatched as data messages. The data may be combined into one or more packets, and in this combined form are sent over the network connections to the respective receiver. Hence, this combined form of data is sometimes termed “data packets.” The term “transmission of data” as used hereinafter is synonymous with the above-described transmission of data messages or data packets.
Users of a communication system are, for example, central automation devices, programming, configuration and operator units, peripheral devices such as, for example, input/output modules, drives, actuators, sensors, stored program controllers (SPCs) or other control units, computers or machines that exchange electronic data with other machines, and in particular, process data of other machines. Users are also referred to as network nodes or nodes.
The term “control units”, as used hereinafter, refers not only to open and closed-loop control units of all types but also to, for example, coupling nodes (switches) and/or switch controllers. For example, switched Ethernets, industrial Ethernets and particularly, isochronous Real-Time Ethernets are used as communication systems or data networks.
Communication systems with transfer rates ≧100 MB/s are usually switched high-performance data networks and are made up of individual point-to-point connections with so-called active nodes or coupling nodes, also referred to as switches, which are switched between the separate users. As a rule, each coupling node has a plurality of ports. Each coupling node can be connected to a number of users depending on the number of ports. A switch may itself be a user.
These coupling nodes or switches are currently separate devices. These coupling nodes, however, are increasingly integrated directly into the connected devices or users. Such a cost-effective integration of the switches or the switch functionality into the individual users or automation devices is made possible by VLSI technology (Very Large Scale Integration technology). Thus, with suitable cabling, communication systems can have substantially larger dimensions because only user-to-user connections are employed.
In these communication systems with a very large number of users, particularly in linear arrangements, the active node or switch is a potential source of problems. If a user fails, the entire line fails. In addition, if an additional user is inserted, the communication system must be briefly interrupted. Closing the line into a ring with a redundant path would be desirable.
If the communication system is designed in a ring, in the event of a fault in a network connection, for example during an interruption or the failure of a user, the data traffic could alternatively be routed through the remaining network connections. However, in automatic destination finding and/or broadcast/multicast addressing such redundant paths lead to circulating data messages. To prevent these continuously circulating data messages, when a communication system is started up, a check for possible circularities is performed, and the communication system is logically interrupted at a suitable location. That is, although the communication system is connected in a ring, a suitable location for a stop point is created. The redundant path is used only in the standby mode. In other words, in the event of faults, the system switches to the backup paths within the context of a reconfiguration. However, the outage caused by this reconfiguration cannot be tolerated in many automation applications.
A method for the identification of circularities in a communication system is known or prescribed by the IEEE 802.1Q standard. This method is also called the “spanning tree” or the “fast spanning tree” method. The simpler the topology of a communication system or the smaller the communication system, the faster a fault is detected, and the communication system is dynamically reconfigured. This reconfiguration again includes a check for circularities within the communication network and depending on the topology and size of the network may last into the double-digit seconds range. For a ring-type communication system, a dynamic reconfiguration time of less than 1 second is possible if special methods are used. Durations of this magnitude are currently acceptable in the fields of application of communication networks of this type.
With the current methods, however, when faults occur, completely bumpless or transparent switching over time is not possible. To ensure bumpless and transparent switching, data messages must be permanently redundantly transmitted on disjunctive network paths. Such a permanent use of redundant network paths is possible if the full path information of a data message transfer is stored in advance in all users involved in the communication system. Such a method is known, in particular, for isochronous Real-Time Ethernet communication from DE 10058524.8. In contrast, in data communication with independent path search, that is, in an address-based data communication, similar solutions are not known.